Even with the variety of measures now being taken and efforts that have been and continue to be made, air contamination continues to be of major concern, and is a problem that continues to grow, especially in urban and industrial areas. In the case of domestic and industrial burners that operate on middle distillate fuels, despite the progress that has resulted from the development of so-called blue burners which tend to emit lower levels of nitrogen oxides (NOx) than the prior so-called yellow burners, further improvements in operational efficiency and reductions in emissions in the flue gas would be a most welcome contribution to the art. Blue burners are generally designed and in many cases calibrated to operate with excess air in the range of 5 to 15% excess air over the stoichiometric (chemically equivalent) amount of air needed to burn the fuel as it is being burned in the combustion zone(s). In other words, the air intake is regulated so that the oxygen content of the air being fed to the combustion process is in the range of 5 to 15% more than the exact minimum quantity theoretically required to burn the amount of fuel being fed to the combustion process.
It has been found heretofore that manganese polycarbonyl compounds are effective in reducing smoke and soot produced on burning fuel oil in earlier types of domestic fuel oil burners. See for example U.S. Pat. No. 3,112,789 to Percy et al. which, on the basis of studies conducted with a Timken wall-flame burner, recommends operation with fuel oils containing 0.00125 to 0.005% of manganese as oil-soluble indenyl manganese tricarbonyl, cyclopentadienyl manganese tricarbonyl, and alkyl derivatives thereof using 125 to 140% of the stoichiometric amount of air. Also of interest in this connection is published European Patent Application No. EP 0 476 197A (published in March, 1992) which describes test results obtained using a domestic heating gas oil in two different burners. One was a modern burner whereas the other was a burner produced over fifteen years earlier. Both burners were adjusted to manufacturer's specifications and operated on the clear base fuel and on the same fuel to which had been added an additive formed from methylcyclopentadienyl manganese tricarbonyl along with other components such as overbased calcium sulfonate, ashless dispersant, corrosion inhibitor, metal passivator and demulsifier. Other documents of general background interest cited and abstracted in EP 0 476 197A are: Keszthelyi et al., Period. Polytech., Chem. Eng., Volume 21(1), pages 79-93 (1977); Margantsevye Antidetonatory, edited by A. N. Nesmeyanov, Nauka, Moscow, 1971, pages 192-199; Zubarev et al., Rybn. Khoz. (Moscow), Volume 9, pages 52-4 (1977); Canadian Patent No. 1,188,891; EP Patent No. 0078249 B1; GB Patent No. 1,413,323, and to a lesser extent, U.S. Pat. No. 4,505,718.